The present invention relates to electrical discharge machining (EDM) in general and more particularly to a method and apparatus for cutting a workpiece in a travelling wire EDM apparatus.
Internal stresses appearing in an electrically conductive workpiece in the process of being cut in a travelling wire EDM apparatus by means of an electrode wire is a common occurrence. The workpiece is severed in two portions which remain interconnected by a solid bridge of uncut metal. At the end of the cut, the bridge of uncut metal becomes progressively thinner, and internal stresses appear in the workpiece due to the pull exerted by the almost cut-off portion of the workpiece through the bridge of material.
Those internal stresses cause a deformation of the workpiece which is the cause of some machining errors. In addition, some specific precautionary measures must be taken to prevent the fall of the cut-out portion when the bridge of material is electrically eroded in its entirety Complete automation of the cutting operation is, under those conditions, very difficult to achieve.
Several solutions have been proposed in the past to prevent the fall of the cut-off portion of the workpiece, for example by providing a support plate sliding under the workpiece, or a pivoting support arm under the workpiece displaced by the same mechanism as the mechanism displacing the wire support and guide members, or yet by means of wedges placed in the cut or cleft.
All those solutions are complex and they require the intervention of the machine operator In addition they are not suitable to a completely automatic cutting operation.